The invention generally relates to a device for degassing the condensate in the cycle of an electricity generating plant whose main cycle consists essentially of steam generator, turbine, condenser, low pressure and high pressure preheaters and the necessary circulating pumps.
Oxygen dissolved in the feed-water, dissolved copper and other corrosion products carried by the feed-water have been identified as the agents promoting corrosion in the steam generators of pressurized water reactor plants. For this reason, great efforts have recently been made to keep the oxygen content in the feed-water as low as possible, to eliminate the dissolved copper by the replacement of every copper alloy tube in the feed-water circuit and to separate corrosion products before they reach the steam generator. This is all the more important because the replacement of the steam generator for a power station of the 1,000 MW class leads to costs of approximately 100 million dollars. Reduction of the oxygen content in the feed-water also reduces the corrosion of the feed-water train and the preheaters.
Oxygen is dissolved in the feed-water or in the condensate whenever air comes in contact with the water. This is, for example, the case:
during cold starting of a plant because all the parts of the plant are under air pressure before they are filled with condensate and it is not possible to remove all the air from the feed-water/steam circuit by the start-up evacuation; PA1 during low-load operation because the suction capacity of the vacuum pumps for the air introduced cannot, from economic considerations and for reasons of feasibility, be selected sufficiently large for all parts of the condenser bundle to be adequately flushed with steam; PA1 during normal load operation because, despite adequate suction capacity, the air concentration becomes so great towards the end of condensation, i.e. towards the air cooler, that measurable oxygen concentrations result; PA1 because even during normal load operation, many condenser designs have zones in which air collects; PA1 because during the treatment of make-up water, the cleaned desalinated make-up water is washed through with air in order to drive out carbon dioxide and is therefore 100% saturated with air.
Copper is, for example, dissolved from the wetted metal surfaces of copper alloys in the presence of ammonia and oxygen or is carried into the feed-water by erosion or corrosion of these metal surfaces. It is mainly during outage periods that corrosion products and other impurities collect in the feed-water.
Specialists are currently of the opinion that a maximum oxygen content of less than or equal to 10 ppb (part per billion) should be the target value over the whole of the operating range of the plant.
In good condensers, i.e. those with a proven good degassing capability, oxygen contents of less than or equal to 5 ppb have been found in the condensate in the load range between 40 and 100%. The make-up water is then degassed in the condenser itself. During a cold start of such a plant and in low-load operation, however, about 70 ppb has been measured in the hot well of the condenser. These oxygen contents would have to be further reduced.
The following proposals for solving the corrosion problem were discussed at the EPRI Condenser Seminar in June 1983 in Orlando, Fla.:
(a) Start-up degassing and low-load degassing by spraying recirculated feed-water over the tubes of the condenser. This idea only leads to success if the available suction capacity is larger than the suction capacity necessary for oxygen contents of less than or equal to 10 ppb. This can only be expected at loads from 30 to 40%. This is because spraying heated condensate over the condenser bundle only has the desired effect if the condensate does not have to pass through zones of larger air concentrations at any part of the path traversed by the condensate. Furthermore, it is impossible to clean the whole of the feed-water circuit by condensate recirculation.
(b) Increasing the suction capacity in the low-load range by reducing the steam content of the suction flow using condensation by mixing. Condensation by mixing, however, can only condense part of the water vapour from the suction mixture; the suction unit must still remove all the air and this is generally only possible at condenser pressures which are above the "idling pressure" of the condenser.
(c) Extra degassing of the condensate in the hot well by means of included drip features. In the case of such drip features, sufficient height must be available for their inclusion.
(d) Extra degassing of the condensate in the hot well by blowing in steam under the water level. Steam blowing requires a sufficiently large covering of condensate and a sufficiently fine steam distribution in the condensate.